1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus such as color photocopier and a color printer, and more particularly to an apparatus for supplying a high level of voltage to a developing device for a color printing or copying.
2. Description of the Related Art
As shown in FIG. 1, an electrophotographic color image forming apparatus such as a color laser printer or a color photocopier is generally equipped with a photosensitive medium 11, usually in a drum type, which is continuously rotated in a certain direction by a photosensitive medium driving source (not shown).
Along the outer circumference of the photosensitive medium 11, there are provided in the direction of rotation by order of: a charging unit 12; a laser scanning unit (LSU) 20; four sliding developing devices 31, 32, 33, 34 respectively storing yellow, magenta, cyan and black developers; a transfer unit 60; a discharge lamp 87; and a cleaning discharge unit 80.
The charging unit 12 is a scorotron charger, which functions to evenly charge the photosensitive medium 11. The LSU 20 irradiates in an axial direction a laser beam of a linear form onto the photosensitive medium 11.
The respective developing devices 31, 32, 33, 34 is either a thickness restricting member or a blade 51, which restricts a thickness of developer layer accumulating on the components such as a developing roller 13, a developer receiving unit 16, a developer feeding roller 15 and a developing roller 13. The developing devices 31, 32, 33, 34 are rotated by a development driving source (not shown). The developer is fed to the developing roller 13 via the developer feeding roller 15 of the developer receiving unit 16 which is applied with a predetermined high voltage, and is controlled to under a predetermined thin layer on the developing roller 13 by the thickness restricting blade 51.
The developing devices 31, 32, 33, 34 are a sliding type developing device, and supported to reciprocate inside a developing device guiding member (not shown). The developing devices 31, 32, 33, 34 are moved toward the photosensitive medium 11, overcoming the spring 74, by the eccentric cams 35 36, 37, 38 which are respectively fixed to a rotation shaft 56. Rotation of the rotation shaft 56 is controlled by an electric clutch (not shown).
During developing process, a developing bias voltage is applied to the developing roller 13. In case of negative-positive inversion, the developing bias voltage has the same polarity as that of the charging voltage of the photosensitive medium 11.
The transfer unit 60 electrostatically transfers the color image from the photosensitive medium 11 onto a recording paper (P), and the cleaning discharging unit 80 removes any residual developer from the photosensitive medium 11.
Describing the operation of the image forming apparatus 10 in more detail, first, with a print command, the photosensitive medium 11 is rotated by the photosensitive medium driving source (not shown), and the surface of the photosensitive medium 11 is charged by the charging unit 12 evenly. When the charged area reaches a developing position (d) for a first color to be printed, for example, when the charged area reaches a developing position (d) for the yellow developing device 31, an electric clutch of the yellow developing device is turned on, and therefore, moved toward the photosensitive medium 11 by the eccentric cam 35 and set to developing state.
Next, the surface of the photosensitive medium 11 is exposed to the LSU 20, and thus having a yellow electrostatic latent image formed thereon. As a result, a successive yellow image is developed at the developing position (d) by the developing device 31, from the leading end to the rear end of the image.
After the completion of the yellow image formation, and after the rear end of the image passes the developing position (d), the eccentric cam 35 is rotated, and accordingly, the yellow developing device 31 is separated from the photosensitive medium 11.
Then as the leading end of the image reaches a formation position (e) of the second color image, for example, when the leading end of the image reaches the formation position (e) of the magenta developing device 32, the electric clutch of the magenta developing device 32 is turned on, and as a result, the magenta developing device 32 is set to the developing state by the eccentric cam 36.
At this time, the yellow image being formed on the photosensitive medium 11, is passed through the transfer unit 60, the discharge lamp 87 and the cleaning discharge unit 80, all of which being not in operation, and then positioned below the charging unit 12. In order not to blur the image passing therebetween, usually, the transfer unit 60 and the cleaning discharging unit 80 are spaced apart from each other except for when the both are in operation.
The photosensitive medium 11, now being formed with the second color image, i.e., the yellow image under the charging unit 12, is again charged by the charging unit 12 evenly. Then the image corresponding to the magenta color is overlappingly formed by the exposure to the LSU 20, and the magenta image is developed by the magenta developing device 32 at the magenta developing position (e). After the completion of the magenta image formation and when the rear end of the image is passed through the magenta image developing position (e), the eccentric cam 36 is rotated, and as a result, the magenta developing device 32 is separated from the photosensitive medium 11.
Next, when the rear end of the image reaches a developing position (f) for the third color image, for example, when the rear end of the image reaches a developing position (f) for the cyan developing device 33, the electric clutch of the cyan developing device 33 is turned on, and by the eccentric cam 37, the cyan developing device is set to a developing state.
The composite image of yellow and magenta images, which has passed through the transfer unit 60, the discharging lamp 87 and the cleaner discharging unit 80, is positioned under the charging unit 12 again, and the photosensitive medium 11 is charged by the charging unit 12 evenly. The yellow-magenta image is overlapped with the cyan image, by the LSU 20 (32-4), and developed at the cyan developing position (f) by the cyan developing device 33. When the cyan image is formed, the rear end of the image passes through the cyan developing position (f), and the eccentric cam 37 is rotated so that the cyan developing device 33 is separated from the photosensitive medium 11.
Next, the black image is overlapped and formed in the same way as described above, and as a result, the image formation is completed. When being completed, the color image on the photosensitive medium 11 is transferred onto the recording paper P which is conveyed from the recording paper feeding unit.
After the transfer, the photosensitive medium 11 is discharged by the discharging lamp 87, and by the rotatable brush 81 of the cleaner discharging unit 80, a residual developer on the surface of the photosensitive medium 11 is removed so that the photosensitive medium 11 is returned to the initial state. The recording paper P with the image formed thereon is conveyed to a recording paper fusing unit and thus image is firmly attached to the paper and discharged out.
As described above, in a conventional image forming apparatus 10, approximately four developing devices 31, 32, 33, 34 representing the respective colors are constructed such that the same are slid to contact the photosensitive medium 11 by a predetermined pressure, or be spaced apart from the photosensitive medium 11, by the operation of the eccentric cams 35, 36, 37, 38, respectively. During one rotation of the photosensitive medium 11, i.e., in development of one sheet of recording paper, the four color developing devices 31, 32, 33, 34 are contacted with the developing roller 13 respectively once, and therefore, the developing devices 31, 32, 33, 34 are contacted with the developing roller 13 four times in total. As shown in FIG. 2, voltage supply sliding contact terminals 13a, 15a, 51a, which are connected with the developing roller 13, the developer feeding roller 15 and the developer layer thickness restricting blade 51, are sequentially connected with, or disconnected from fixed contact point terminals 90a′, 90b′, 90c′. The fixed contact point terminals 90a′, 90b′, 90c′ are connected with corresponding voltage units of the high voltage power supply (HVPS) 90, i.e., a developing roller voltage supply (supply, 90a), a developer feeding roller voltage supply (Deve, 90b), and a developer layer thickness restricting blade voltage supply (Blade, 90c) through a wiring harness.
However, the conventional image forming apparatus 10 has a problem of complex construction due to requirement for the eccentric cams 35, 36, 37, 38, cam driving motor (not shown) and the electric clutch for the changing of the developing device among the four developing devices 31, 32, 33, 34.
Further, in every changing of the developing device, the impact of the contact between the photosensitive medium 11 and the developing roller 13 of each developing device 31, 32, 33, 34 is directly transmitted to the photosensitive medium 11, and as a result, the lifetime of the photosensitive medium 11 is shortened. Additionally, the impact from the contact also causes change of running speed of the photosensitive medium 11, which subsequently causes a degradation of printing quality such as a ‘jitter’.
Further, because of the complex structure in which the voltage supply device for the developing devices 31, 32, 33, 34 are connected with the voltage units 90a, 90b, 90c and the fixed contact point terminals 90a′, 90b′, 90c′ of the high voltage supply 90 through a rather complex wiring harness, fabricating is complicated. Also, due to sliding contact between the sliding contact point terminals 13a, 15a, 51a and the fixed contact point terminals 90a′, 90b′, 90c′, reliability of high voltage contact point changeover is deteriorated.